Magnetic Field Deflector in an Induction Resistivity Tool

ABSTRACT

A downhole induction resistivity assembly comprises a mandrel. Disposed around the mandrel are coils of wire disposed circumferentially around magnetic field concentrators. The coils of wire and the magnetic field concentrators are disposed on an outer diameter of the mandrel. A magnetic field deflector, of magnetic permeability greater than the mandrel, is disposed intermediate the coils and the mandrel.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/550,501, which is a continuation-in-part of U.S. patentapplication Ser. No. 12/473,416, which is a continuation-in-part of U.S.patent application Ser. No. 12/341,771 filed on Dec. 22, 2008, which isa continuation-in-part of U.S. patent application Ser. No. 11/776,447filed on Jul. 11, 2007 which claims priority to Provisional U.S. PatentApplication No. 60/914,619 filed on Apr. 27, 2007 and entitled“Resistivity Tool.” This application is also a continuation-in-part ofU.S. patent application Ser. Nos. 11/676,494; 11/687,891; 61/073,190.All of the above mentioned references are herein incorporated byreference for all that they contain.

BACKGROUND OF THE INVENTION

Electric resistivity of a downhole formation is often measured from awireline in a well bore to analyze formation parameters. Inductionresistivity tools induce a magnetic field into the formation; and thus,are different from laterolog resistivity systems, where an electriccurrent is passed through the formation.

U.S. Pat. No. 6,677,756 to Fanini, et al, which is herein incorporatedby reference for all that it contains, discloses an induction tool forformation resistivity evaluations. The tool provides electromagnetictransmitters and sensors suitable for transmitting and receivingmagnetic fields in radial directions.

U.S. Pat. No. 6,359,438 to Bittar, which is herein incorporated byreference for all that it contains, discloses a resistivity tool for usein an LWD system that includes a transmitter array with multipletransmitters positioned above a pair of receivers. The transmitters areselectively energized, causing current to be induced in the collar ofthe tool.

U.S. Pat. No. 6,577,129 to Thompson, et al, which is herein incorporatedby reference for all that it contains, discloses an electromagnetic wavepropagation resistivity borehole logging system comprising multiplegroups of electromagnetic transmitter-receiver arrays operating at threefrequencies.

U.S. Pat. No. 6,538,447 to Bittar, which is herein incorporated byreference for all that it contains, discloses a multi mode resistivitytool for use in a logging-while-drilling system that includes anasymmetric transmitter design with multiple transmitters capable ofgenerating electromagnetic signals at multiple depths of investigation.

U.S. Pat. No. 7,141,981 to Folbert, et al, which is herein incorporatedby reference for all that it contains, discloses a resistivity loggingtool suitable for downhole use that includes a transmitter, and twospaced apart receivers. The measured resistivities at the two receiversare corrected based on measuring the responses of the receivers to acalibration signal.

U.S. Pat. No. 6,218,842 to Bittar, et al, which is herein incorporatedby reference for all that it contains, discloses a resistivity tool foruse in LWD systems that includes an asymmetric transmitter design withmultiple transmitters capable of generating EM signals at multiplefrequencies.

U.S. Pat. No. 5,045,795 to Gianzero, et al, which is herein incorporatedby reference for all that it contains, discloses a coil array which isinstalled on a MWD drill collar for use in a resistivity logging system.The drill collar is provided with upper and lower coil support rings.These are toroids which support individual coil segments, and areconnected by suitable magnetic shorting bars. The coil segments andshorting bars inscribe a specified solid angle or azimuthal extent.

U.S. Pat. No. 5,606,260 to Giordano, et al, which is herein incorporatedby reference for all that it contains, discloses a microdevice isprovided for measuring the electromagnetic characteristics of a mediumin a borehole. The microdevice includes at least one emitting ortransmitting coil (31), and at least one receiving coil (41,51). Themicrodevice generates an A.C. voltage at the terminals of thetransmitting coil and measures a signal at the terminals of thereceiving coil. The microdevice also includes an E-shaped electricallyinsulating, soft magnetic material circuit serving as a support for eachof the coils and which is positioned adjacent to the medium in theborehole.

U.S. Pat. No. 6,100,696 to Sinclair, which is herein incorporated byreference for all that it contains, discloses a directional inductionlogging tool that is provided for measurement while drilling. This toolis preferably placed in a side pocket of a drill collar, and itcomprises transmitter and receiver coils and an electromagneticreflector.

U.S. Pat. No. 6,163,155 to Bittar, et al, which is herein incorporatedby reference for all that it contains, discloses a downhole method andapparatus for simultaneously determining the horizontal resistivity,vertical resistivity, and relative dip angle for anisotropic earthformations.

U.S. Pat. No. 6,476,609 to Bittar, et al, which is herein incorporatedby reference for all that it contains, discloses an antennaconfiguration in which a transmitter antenna and a receiver antenna areoriented in nonparallel planes such that the vertical resistivity andthe relative dip angle are decoupled.

U.S. Pat. No. 6,900,640 to Fanini, et al, which is herein incorporatedby reference for all that it contains, discloses a tool that provideselectromagnetic transmitters and sensors suitable for transmitting andreceiving magnetic fields in radial directions that are orthogonal tothe tool's longitudinal axis with minimal susceptibility to errorsassociated with parasitic eddy currents induced in the metal componentssurrounding the transmitter and receiver coils.

BRIEF SUMMARY OF THE INVENTION

In one aspect of the present invention a downhole induction resistivityassembly comprises a mandrel. Disposed around the mandrel are coils ofwire disposed circumferentially around magnetic field concentrators. Thecoils of wire and the magnetic field concentrators are disposed on anouter diameter of the mandrel. A magnetic field deflector, whichcomprises a magnetic permeability greater than the mandrel, may bedisposed intermediate the coils and the mandrel. The magnetic fieldconcentrator may comprise a ferrite core.

In some embodiments of the invention, the magnetic field deflector maycomprise a mu-metal, Hipernum, HyMu-80, permalloy, a magnetically softalloy or sheet metal, or any material or alloy with a permeabilitygreater than the mandrel. The deflector material may contain nickel,iron, manganese, molybdenum, silicon, magnetic material, carbon, or anycombination thereof. The material may further comprise an alloy thatcomprises between 70% to 85% nickel and between 10% to 20% iron. Thedeflector may be annealed. The deflector may comprise a material with ahigher permeability value than the mandrel. The material my furthercomprise having a magnetic permeability that is at least 100 timesgreater than the permeability of the mandrel.

In some embodiments, the magnetic field deflector may be disposedcircumferentially at least once around the mandrel. The deflector may beintermediate transmitter coils, bucking coils, or receiver coils. Thedeflector may be disposed under a portion of the coils. The positioningof the deflector may comprise spanning a space between a transmittercoil and a receiver coil. The deflector may be disposedcircumferentially around the entire length of the mandrel. The deflectormay comprise a sheet of material or wire/cable wrapper circumferentiallyaround the mandrel. Strips of a magnetic deflector may be disposedcircumferentially around the mandrel adjacent to an end of a receivercoil.

In some embodiments, the mandrel may comprise a magnetic material. Themandrel may further comprise sections of magnetic and nonmagneticmaterial. A magnetic section may be position proximate a transmitter orreceiver.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional diagram of an embodiment of a downholeinduction resistivity assembly.

FIG. 2 is a perspective diagram of an embodiment of inductionresistivity assembly.

FIG. 3 a is a perspective diagram of an embodiment of an inductiontransmitter.

FIG. 3 b is a cross-sectional diagram of an embodiment of an inductiontransmitter.

FIG. 4 a is a perspective diagram of a downhole tool string.

FIG. 4 b is a perspective diagram of an embodiment of a magnetic fielddeflector.

FIG. 5 a is a perspective diagram of an embodiment of an inductiontransmitter or receiver.

FIG. 5 b is a perspective diagram of an embodiment of a magnetic fielddeflector.

FIG. 6 is a perspective diagram of an embodiment of a magnetic fielddeflector.

FIG. 7 is a perspective diagram of an embodiment of a magnetic fielddeflector.

FIG. 8 is a cross-sectional diagram of an embodiment of a magnetic fielddeflector.

FIG. 9 is a cross-sectional diagram of an embodiment of a magnetic fielddeflector.

FIG. 10 is a perspective diagram of an embodiment of a magneticdeflector disposed on a mandrel.

DETAILED DESCRIPTION OF THE INVENTION AND THE PREFERRED EMBODIMENT

Referring now to FIG. 1, a downhole tool string 101 may be suspended bya 10 derrick 102. The tool string may comprise one or more downholecomponents 100, linked together in a tool string 101 and incommunication with surface equipment 103 through a downhole network. Thenetwork may enable high-speed communication between devices connected tothe tool string, and the network may facilitate the transmission of databetween sensors and sources. The data gathered by the downholeinstrumentation may be processed downhole, may be transmitted to thesurface for processing, may be filtered downhole and then transmitted tothe surface for processing, may be compressed downhole and thentransmitted to the surface for processing or combinations thereof. Insome embodiments, the data may be stored downhole and dumped to upholdequipment when the tool string is tripped out of the wellbore.

FIG. 2 is an embodiment of a tool string component 100. The tool stringcomponent may comprise an induction transmitter 201 and a plurality ofinduction receivers 202 and 203. The receivers 202 and 203 may be placedin a variety of orientations with respect to each other and to thetransmitter 201. The induction transmitter 201 is adapted to send aninduction signal into the formation, which generates an induced field inthe formation surrounding the well bore. The induction receivers 202 and203 are adapted to sense various attributes of the induced formationfield. These attributes may include among others, some or all of thefollowing: frequency, amplitude, or phase. The transmitter and thereceivers may be powered by batteries, a turbine generator, or from thedownhole network. The receivers may also be passive. In someembodiments, there may be several induction transmitters located alongthe length of the tool string component. In some embodiments, theadditional transmitters may be used to calibrate measurements, such asis common in borehole compensation techniques.

FIG. 3 a is a perspective diagram of an embodiment of an inductiontransmitter. The transmitters 300 may comprise coils of electricallyconductive material 304 wrapped around a magnetic field concentrator305. The coil wrapped field concentrators 305 may be disposed on theouter diameter of a mandrel. The mandrel may be a drill pipe, toolstring component, or combination thereof. The magnetic fieldconcentrator 305 may comprise a magnetically conducting core where awire is wrapped around its diameter or a magnetically conducting annularring where the wire is disposed within a ring's recess.

FIG. 3 b is a cross-sectional diagram of an embodiment of an inductiontransmitter. The transmitter coils 301 may be disposed circumferentiallyaround the mandrel 302. A magnetic field deflector 303 may be disposedcircumferentially intermediate the mandrel 302 and the transmitter coils301. The magnetic field deflector 303 may comprise a material that has ahigher magnetic permeability than that of the mandrel 302. The deflector303 may comprise mu-metal, Hipernum, HyMu-80, permalloy, a magneticallysoft alloy or sheet metal. In some embodiments, the deflector 303 maycomprise any material that has a relative magnetic permeability at least100 times greater than the magnetic permeability of the mandrel 302. Thedeflector 303 may comprise alloys of iron and nickel. The deflector 303may be annealed. Annealing of the magnetic field deflector increases themagnetic permeability of the material by aligning the grains of themetal. The higher permeability may provide a path of least resistancefor a magnetic field around the mandrel, thereby shielding the mandrelfrom the magnetic field. In some embodiments, the deflector may besegmented.

As the coils of the transmitters carry an electric current, itsassociated magnetic field is concentrated by the magnetic concentrators.The coils and magnetic concentrators work together to control theparameters of the induced field. The deflector is disposed morecentrally to the mandrel's central axis than both the coils and themagnetic concentrators and may insulate, or even isolate, the mandrelfrom the induced magnetic field. Preferably, the coils are wrappedaround the magnetic field concentrator.

FIG. 4 a is a perspective diagram of a downhole tool string component.As an induction transmitter 400 produces a magnetic field 401, a currentis induced in the mandrel 302. The induced current 402 can be measuredby a spectrum analyzer 403, which may be attached to the ends of themandrel 302. The current 403 in the mandrel 302 produces a magneticfield that can be picked up by the receiver coils on the drill pipe,thereby interfering with acquiring resistivity measurements.

FIG. 4 b discloses a magnetic field deflector positioned intermediatethe transmitter coils 400 and the mandrel 302 that reduces or eliminatesthe induced current in the mandrel 302. The spectrum analyzer 403 maynot detect a current 402 induced on the mandrel 302. Decreasing oreliminating the magnetic field produced by the mandrel 302 increases thesensitivity of the receiver coils to magnetic fields emanating from theformation.

FIG. 5 a is a perspective diagram of an embodiment of an inductiontransmitter or receiver. The induction transmitter or receiver 501 maycomprise wrapping an electrically conductive wire 502 around a magneticfield concentrator 503. The magnetic field concentrator 503 may comprisea ferrite core. As the coil of wires induce a magnetic field, the fieldis concentrated by the ferrite core 503, and the field is concentratedin the core 503.

FIG. 5 b is a perspective diagram of an embodiment of a magnetic fielddeflector 504. The magnetic field deflector 504 may be disposescircumferentially around the mandrel 302. The magnetic field deflector504 may be disposed intermediate at least one coil 502 and the mandrel302. The deflector 504 may span more or less area than the area underthe transmitter or receiver coils 501.

FIG. 6 is a perspective diagram of an embodiment of a magnetic fielddeflector 601. In this embodiment the deflector 601 may comprise a wireor cable 601 wrapped circumferentially at least once around the mandrel302. The deflector 601 may be disposed intermediate the inductiontransmitter or receiver 501 and the mandrel 302. The deflector 601 coilsmay be spaced apart or tightly wound. In some embodiments, the wire orcable turns are in electrical communication with each other so theentire deflector acts as a sheet.

FIG. 7 is a perspective diagram of an embodiment of a magnetic fielddeflector. A deflector 701 may be disposed circumferentially around themandrel 302 such that the deflector 701 is adjacent the ends of theinduction transmitter or receiver 501.

FIG. 8 is a cross-sectional diagram of an embodiment of a magnetic fielddeflector. A transmitter coil may comprise a bucking coil 801. Thebucking coil 801 induces a magnetic field 802 which pushes a magneticfield 803 produced by other transmitter coils 804 into the formation805. A deflector 303 may be disposed intermediate the mandrel 302 andthe bucking coil 801.

FIG. 9 is a cross-sectional diagram of an embodiment of a magnetic fielddeflector 303. The magnetic field lines 1001 depicted in diagram willfollow a path a least resistance. The magnetic field deflector 303comprises a material of higher magnetic permeability than the mandrelsuch that the magnetic field lines 1001 travel preferentially throughthe deflector 303 instead of the mandrel 302.

FIG. 10 is a perspective diagram of an embodiment of a magnetic fielddeflector 1101 disposed on a mandrel 303. The deflector 1101 may bedisposed circumferentially around the mandrel 303 at least once. In thefigure multiple layers are depicted. Additional layers provide moreshielding and would decrease the induced field in the mandrel 303.

In some embodiments, the magnetic deflector may be electrically isolatedfrom the mandrel. The magnetic deflector may also be segmented axiallyor circumferentially around the outer diameter of the mandrel.

Whereas the present invention has been described in particular relationto the drawings attached hereto, it should be understood that other andfurther modifications apart from those shown or suggested herein, may bemade within the scope and spirit of the present invention.

1. A downhole induction resistivity assembly, comprising: a mandrel; acoil of wire disposed circumferentially around a magnetic fieldconcentrator; wherein the coil and magnetic field concentrator aredisposed on an outer diameter of the mandrel; a magnetic field deflectordisposed intermediate the mandrel and the coil; wherein the magneticfield deflector comprises a magnetic permeability greater than that ofthe mandrel.
 2. The assembly of claim 1, wherein the magnetic fielddeflector may comprise a mu-metal, Hipernum, HyMu-80, permalloy, amagnetically soft alloy or sheet metal.
 3. The assembly of claim 1,wherein the magnetic field deflector comprises an alloy which maycontain nickel, iron, manganese, molybdenum, silicon, or carbon.
 4. Theassembly of claim 3, wherein the alloy comprises between 70% to 85%nickel and between 10% to 20% iron.
 5. The assembly of claim 1, whereinthe magnetic field deflector comprises a magnetically conductivematerial that is annealed.
 6. The assembly of claim 1, wherein themagnetic field deflector is disposed circumferentially at least oncearound the mandrel.
 7. The assembly of claim 1, wherein the magneticfield deflector is disposed intermediate a bucking coil and the mandrel.8. The assembly of claim 1, wherein a magnetic field line generated by atransmitter wraps around the mandrel through the magnetic fielddeflector.
 9. The assembly of claim 1, wherein the magnetic fielddeflector is disposed circumferentially around the mandrel spanning aspace between a transmitter coil and a receiver coil.
 10. The assemblyof claim 1, wherein the magnetic field deflector comprises a sheetdisposed circumferentially around the mandrel.
 11. The assembly of claim1, wherein the magnetic field deflector is disposed under a portion ofthe coils.
 12. The assembly of claim 1, wherein the magnetic fielddeflector is disposed circumferentially around the entire length of themandrel.
 13. The assembly of claim 1, wherein the mandrel comprises amagnetic material.
 14. The assembly of claim 15, wherein the mandrelcomprises sections of magnetic and nonmagnetic material.
 15. Theassembly of claim 16, wherein the magnetic section of the mandrel ispositioned proximate a transmitter or receiver.
 16. The assembly ofclaim 1, wherein the magnetic field deflector is disposedcircumferentially around the mandrel adjacent to an end of a receivercoil.
 17. The assembly of claim 1, wherein the magnetic field deflectorcomprises a material with a higher mu value than the mandrel.
 18. Theassembly of claim 1, wherein the coil adjacent the mandrel comprisestransmitter coils and receiver coils.
 19. The assembly of claim 1,wherein the magnetic field concentrator comprises a ferrite core.